1. Field of the Invention
This invention relates to a mold for the manufacture of golf balls. More particularly, it relates to a mold capable of manufacturing golf balls featuring improved flight performance, symmetry and cost performance.
2. Prior Art
For golf balls, it is desired that dimples be equally distributed on the spherical surface so that the ball may exhibit equal flight performance independent of points of a contact for a shot. That is, golf balls which are improved not only in flight performance, but also in symmetry are required. The requirement now becomes more outstanding as the golf game becomes more popular to the public, the golfer population increases, and the demand of golfers is diversified.
On the other hand, to supply high performance, high quality golf balls at a reasonable price, it is necessary to increase the cost performance of golf balls, that is, to reduce the expense invested in the development of golf balls.
Various attempts have been made to develop a high performance, high quality, low price golf ball meeting the demand of consumers.
For example, optimizing the dimple arrangement and dimple shape (including diameter, depth, and cross-sectional shape) leads to an improvement in symmetry and hence, an improvement in flight performance. Attempts have been made to improve the dimple arrangement and shape.
With respect to the arrangement of dimples on a golf ball, various dimple arrangement designs including icosahedral and dodecahedral arrangements have been proposed. Since the mold for use in the manufacture of golf balls typically consists of at least two split mold sections, golf balls regardless of the dimple arrangement generally have at least one great circle which corresponds to the parting line of the mold and which does not intersect with the dimples.
More particularly, golf balls are manufactured using a mold including a pair of mold sections which are removably mated to define a spherical cavity therein. Since the mold sections themselves are molded and processed in an axisymmetric manner, the mold cavity and hence, golf ball tends to possess a higher sphericity or roundness about the pole axis corresponding to a line connecting the apexes of the mold section cavities and inversely, a lower sphericity or roundness about an axis extending in the plane circumscribed by the seam line corresponding to the parting line of the mold. Due to this difference in roundness, conventional golf balls can differ in flight performance depending on the position at which the ball is hit. Such flight performance variation raises a serious problem in the golf game wherein the Rules of Golf prescribes that "the ball shall be played as it lies, except as otherwise provided in the Rules."
More specifically, when a golf ball is hit by a club, the ball is given back spin although the number of revolutions varies with a particular type of club. Ball hitting is generally classified into pole hitting and seam hitting depending on an impact point. Reference is now made to FIG. 9(A) and 9(B) wherein a golf ball 11 has a seam line 12 and a center 16. The pole hitting means that the ball 11 is hit in the direction of an arrow 20 so as to give back spin about a straight line 18 connecting two diametrically opposed points 14, 14 on the seam line 12 and the center 16 as shown in FIG. 9(A). The seam hitting means that the ball 11 is hit in the direction of an arrow 26 so as to give back spin about a straight line 24 extending perpendicular to a circular plane 22 circumscribed by the seam line 12 and passing the center 16. As previously mentioned, in the event of pole hitting shown in FIG. 9(A), the ball is susceptible to extra lift or drag since it does not define a true circle about the spin axis 18. On the other hand, in the event of seam hitting shown in FIG. 9(B), the ball is substantially free of extra lift or drag since it is close to a true circle about the spin axis 24. As a consequence, if the ball is simply designed such that the effect of dimples may be equal between pole hitting and seam hitting, the effect of dimples would be greater on pole hitting because of a deviation from roundness. Then on pole hitting, the golf ball receives extra lift or drag, exhibiting different flight performance than on seam hitting. This means that the flight performance varies with a particular hit position.
Since the golf ball is a unique spherical body deviating little from the complete sphericity as mentioned above, equal dimple effect is not accomplished by uniformly distributing dimples of equal shape on the ball surface, but by additionally finely adjusting the shape of dimples in accordance with a location on the ball surface. It is important to finely adjust the shape of dimples in proximity to the parting line and poles of the ball.
However, the fine adjustment of the dimple shape is established as a result of a trial-and-error procedure involving making a mold for each of candidate dimple shapes, molding a golf ball in the mold, examining the golf ball for flight performance, symmetry or the like, and repeating the steps for all the candidate dimple shapes. This procedure gives rise to the problems that the development period of a golf ball is prolonged and the development cost is increased.
The development of a golf ball generally involves determining an optimum dimple arrangement and finely adjusting or optimizing the dimple shape, especially dimple depth. With a single mold, only a golf ball having a particular dimple shape can be produced. In order to make an adjustment on the dimple shape, a mold corresponding to a new dimple shape is made again, a ball is molded in the mold, and the ball is tested for performance. In this way, a golf ball of a new dimple design is developed by repeating a series of steps of mold making.fwdarw.testing.fwdarw.adjustment.fwdarw.mold making. Cumbersome steps are necessary, the development period is prolonged, the development cost is increased, and hence, the price of a product is increased. With the above-mentioned development procedure, it is virtually difficult to test a number of dimple shapes.